fix: keep the built-in timer presets when a backup has none

A backup without the timer_presets field decoded it as an empty list,
which means "the user removed them all", so restoring it wiped the
built-in presets. A missing field now keeps them. BACKUP.md describes
this and the per-alarm clamps.

app/src/main/java/de/jeanlucmakiola/clockula/data/backup/BackupCodec.kt

@@ -189,7 +189,7 @@ object BackupCodec {
         val settingsDto = settings
         if ((settingsDto.alarmRingtoneUri?.length ?: 0) > MAX_URI_CHARS ||
             (settingsDto.timerRingtoneUri?.length ?: 0) > MAX_URI_CHARS ||
-            settingsDto.timerPresets.size > 1_000
+            (settingsDto.timerPresets?.size ?: 0) > 1_000
         ) {
             return BackupResult.Failure("settings contain oversized values")
         }
@@ -206,7 +206,7 @@ object BackupCodec {
                 dismissChallenge = settingsDto.dismissChallenge.toEnum(DismissChallenge.NONE),
                 homeZoneId = settingsDto.homeZoneId?.let(Zones::normalise),
             ),
-            timerPresets = settingsDto.timerPresets.map { it.milliseconds },
+            timerPresets = settingsDto.timerPresets?.map { it.milliseconds } ?: TimerPresets.BUILT_IN,
         )
 
         return BackupResult.Success(

app/src/main/java/de/jeanlucmakiola/clockula/data/backup/BackupDto.kt

@@ -71,5 +71,10 @@ data class SettingsDto(
     @SerialName("default_timer_ringtone_uri") val timerRingtoneUri: String? = null,
     @SerialName("default_dismiss_challenge") val dismissChallenge: String = "NONE",
     @SerialName("home_zone_id") val homeZoneId: String? = null,
-    @SerialName("timer_presets") val timerPresets: List<Long> = emptyList(),
+    /**
+     * Absent is not empty: an empty list is a user who removed every preset,
+     * while a file without the field (an older or hand-written one) keeps the
+     * built-in ones.
+     */
+    @SerialName("timer_presets") val timerPresets: List<Long>? = null,
 )

app/src/test/java/de/jeanlucmakiola/clockula/data/backup/BackupCodecTest.kt

@@ -257,6 +257,19 @@ class BackupCodecTest {
         assertThat(timers.map { it.duration }).containsExactly(TimerPresets.MIN, TimerPresets.MAX).inOrder()
     }
 
+    @Test
+    fun `a file without timer presets keeps the built-in ones`() {
+        val settings = (BackupCodec.decode(minimalDocument()) as BackupResult.Success).content.settings
+        assertThat(settings.timerPresets).isEqualTo(TimerPresets.BUILT_IN)
+    }
+
+    @Test
+    fun `an empty preset list is honoured as all removed`() {
+        val json = minimalDocument().replace("\"settings\": {}", "\"settings\": {\"timer_presets\": []}")
+        val settings = (BackupCodec.decode(json) as BackupResult.Success).content.settings
+        assertThat(settings.timerPresets).isEmpty()
+    }
+
     @Test
     fun `garbage text fails rather than throwing`() {
         assertThat(BackupCodec.decode("not json at all")).isInstanceOf(BackupResult.Failure::class.java)

docs/BACKUP.md

@@ -38,6 +38,12 @@ written here.
   `ClockPrefs`/`SettingsPrefs` already apply on every write, so a value from
   a future, more permissive version degrades to the nearest valid one instead
   of failing the import.
+- An alarm's own `snooze_minutes`, `snooze_limit` and `volume_ramp_seconds`
+  are clamped at parse time to the same ranges as the defaults they
+  override, and a timer's `duration_millis` to the setup range (1 second to
+  24 hours). Room has no clamp of its own, so this is where it happens.
+- A missing `timer_presets` keeps the built-in presets. An empty list is a
+  user who removed them all, and is imported as such.
 - An enum field with a name this version does not recognise (a `dismiss_
   challenge` or `theme_mode` from a future version, say) degrades to that
   field's default, the same forgiving-read posture `AlarmMapper` already uses